And just how big is El Gordo thought to be? It "has mass about 2 quadrillion ... times that of the sun, making it 'the most massive known cluster in the distant universe.' "

El Gordo is about 7 billion light-years from us. Scientists hope its discovery will help shed some light, as Space.com says, on "dark energy" and "dark matter." And, as the BBC says, it's possible El Gordo will help us figure out how galaxy clusters "form, grow and collide with one another."

NASA adds that "galaxy clusters, the largest objects in the universe that are held together by gravity, form through the merger of smaller groups or sub-clusters of galaxies. Because the formation process depends on the amount of dark matter and dark energy in the universe, clusters can be used to study these mysterious phenomena."

One other note: Scientists say El Gordo's still growing.

Later today, Melissa Block's conversation about El Gordo with astronomer Felipe Menanteau of Rutgers University is due on All Things Considered. Click here to find an NPR station that broadcasts or streams the show.

Finally, this hour, a story that's making us feel quite small. Astronomers using telescopes in Chile have found the largest distant galaxy cluster ever seen. It's nicknamed El Gordo or The Fat One in Spanish. Just how large is it?

It's two with 15 zeros following it in solar masses.

In solar masses. That's Felipe Menanteau of Rutgers University, who led the study that discovered the cluster. And help us understand the size of this galaxy cluster. What is the meaning of that number?

FELIPE MENANTEAU: Well, it's a really, really big number, but what is actually interesting is it is the most massive cluster seen ever in the distant universe. And, you know, there's something neat about being the most massive because, you know, you win the contest right away. But it also - it's giving us an independent way of measuring the current cosmological model in terms of the amount of dark matter and dark energy that makes our universe.

BLOCK: So when you think about this discovery of the largest distant galaxy close to El Gordo that you've found, how does that play into answering some of the big questions that are still out there about the universe, the origin of the universe?

MENANTEAU: Well, one of the remaining questions is the question about the origins and where is everything is coming from, and that's something that cosmology and astronomy are trying to do. So finding El Gordo is confirming our picture of how the universe formed and what is the content in terms of dark energy and dark matter. So what we're doing is that we're actually putting a point there where at - in the very early universe where we believe the structures were formed.

BLOCK: And when we talk about a galaxy cluster, what does that mean? What's happening?

MENANTEAU: So a galaxy cluster is a bunch of galaxies together held by gravity. What I mean of a bunch, we're talking about hundreds of thousands of galaxies. And these are the largest bound structures in the universe, bound by gravity. And these are the densest places of the universe. So clusters of galaxies are interesting for two reasons. One is that we can use them as a signpost in our road for testing the growth of structure of the universe. Another reason why they're so interesting is that, as we're seeing in El Gordo, they're amazing laboratory for astrophysics.

So in El Gordo, we're also seeing emerging clusters. And not long ago, those were two independent very massive clusters that merged and we - actually, we caught them in the act of merging, of crashing against each other.

BLOCK: You caught them as they were colliding.

MENANTEAU: Exactly. We don't know exactly how long ago in terms of in astronomical - in astronomical time, it was very recent. It really looks like a comet. And what we're really seeing is that the hot gas that has been actually stripped from one cluster into the other as one actually cluster is passing through the other.

MENANTEAU: Well, we knew that we have found something unique and extraordinary. I mean, there might be less than a handful of clusters like this in the whole universe, and we wanted to give it a name so people will remember it because in human life, being massive is not something that you're proud of. You're actually always trying to go in the other direction. So we decided to call it El Gordo in order to honor the Chilean connection. And, you know, we were able to get away with calling somebody fat without nobody being offended.

MENANTEAU: I hope so. And, you know, yeah, yeah, we're not going to hear the shouts from here.

BLOCK: I've been talking with astrophysicist Felipe Menanteau of Rutgers University. He led the study that discovered El Gordo, which is the largest distant galaxy cluster ever seen. Thank you so much.

MENANTEAU: Thank you. It was my pleasure.

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